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Local and long-range order in ferroelastic lead phosphate at high pressure.
MedLine Citation:
PMID:  14734839     Owner:  NLM     Status:  PubMed-not-MEDLINE    
Abstract/OtherAbstract:
Pure lead phosphate, Pb(3)(PO(4))(2), undergoes a phase transition from C2/c to R(-)3m symmetry at a pressure of approximately 1.8 GPa and room temperature. Single-crystal X-ray diffraction measurements of the unit-cell parameters of a sample doped with 1.6% Ba(2+) for the Pb(2+) indicates that the doping reduces the transition pressure by approximately 0.1 GPa. The structural evolution of both samples through the phase transition has been determined by Rietveld refinement of neutron powder diffraction data collected to pressures of 6.3 and 3.3 GPa, respectively. There is no evidence for any significant change in the local structure at the phase transition at high pressures; the structure of the R(-)3m phase at pressures just above the phase transition includes disordered positions for several atoms. The observation of diffuse scattering from the R(-)3m phase at high pressure by single-crystal X-ray diffraction suggests that the disorder is static and arises from the presence of several orientations of the ordered microdomains of the monoclinic local structure. The macroscopic transition from monoclinic to trigonal symmetry therefore appears to correspond to the pressure at which the coherency strains between the locally monoclinic microdomains are sufficient to create a dimensionally trigonal lattice within which local displacements of atoms are still significant. A further pressure increase then decreases the magnitude of these displacements until at 3.5 GPa or higher they are not detectable by our current experimental probes, and the structure appears to have true local and global trigonal symmetry.
Authors:
R J Angel; U Bismayer; W G Marshall
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Publication Detail:
Type:  Journal Article     Date:  2004-01-21
Journal Detail:
Title:  Acta crystallographica. Section B, Structural science     Volume:  60     ISSN:  0108-7681     ISO Abbreviation:  Acta Crystallogr., B     Publication Date:  2004 Feb 
Date Detail:
Created Date:  2004-01-21     Completed Date:  2004-06-24     Revised Date:  2007-07-24    
Medline Journal Info:
Nlm Unique ID:  8403252     Medline TA:  Acta Crystallogr B     Country:  Denmark    
Other Details:
Languages:  eng     Pagination:  1-9     Citation Subset:  -    
Affiliation:
Crystallography Laboratory, Department of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24060, USA. rangel@vt.edu
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